Cassiterite Mineral Overview

Mineral name: Cassiterite
Mineral class: Oxide

Known Facts

• Chemical formula: SnO₂
• Crystal system: Tetragonal
• Mohs hardness: 6–7
• Color range: Brown, black, reddish-brown, yellow, colorless
• Luster: Adamantine to vitreous
• Streak: White to light brown
• Cleavage: Poor to indistinct
• Fracture: Conchoidal to uneven
• Specific gravity: Approximately 6.8–7.1
• Common locations: China, Indonesia, Brazil, Bolivia, Democratic Republic of Congo, United Kingdom
• Uses: Primary ore of tin, gemstones, collector specimens
• Similar minerals: Rutile, sphalerite, wolframite

Physical and Optical Characteristics

Cassiterite typically occurs as:

• Prismatic or pyramidal crystals
• Botryoidal (rounded) masses (“wood tin”)
• Granular or massive aggregates

Crystals are often opaque, though gem-quality material can be transparent to translucent.

Color is usually brown to black, but may also be reddish-brown, yellow, or rarely colorless. Some crystals show color zoning.

The luster is often adamantine (very bright and reflective), especially on crystal faces.

A key feature is its very high density, making specimens feel unusually heavy for their size.

Chemical Composition and Structure

Cassiterite has the formula:

• SnO₂

It is a tin oxide, consisting of:

• Tin (Sn⁴⁺)
• Oxygen (O²⁻)

Its structure is similar to that of rutile (TiO₂), with a tetragonal arrangement.

Trace elements such as:

• Iron
• Tantalum
• Niobium

may be present and can influence color and physical properties.

Formation and Geological Occurrence

Cassiterite forms in a variety of geological environments, especially those enriched in tin:

Hydrothermal veins

• One of the most important settings
• Often associated with:
  • Quartz
  • Tourmaline
  • Topaz

Granitic pegmatites

• Forms during late-stage crystallization

Placer deposits

• Due to its high density and resistance to weathering, cassiterite accumulates in:
  • Alluvial (river) deposits

It is commonly associated with:

• Wolframite
• Arsenopyrite
• Fluorite

Common Locations

Major cassiterite deposits are found in:

• China: Largest producer
• Indonesia: Significant placer deposits
• Brazil: Pegmatite and placer sources
• Bolivia: Hydrothermal vein deposits
• Democratic Republic of Congo: Central African tin belt
• United Kingdom: Cornwall (historic tin mining region)

Uses and Practical Significance

Cassiterite is extremely important economically:

Primary tin ore

• Main source of tin (Sn), used in:
  • Solder (electronics)
  • Tin plating (corrosion protection)
  • Alloys (e.g., bronze)

Gemstone use

• Transparent crystals are occasionally cut
• Limited by rarity of gem-quality material

Collecting

• Well-formed crystals and botryoidal varieties are popular

Similar and Related Minerals

Cassiterite may be confused with:

• Rutile: Similar crystal form but lower density and different composition
• Sphalerite: Softer and with different cleavage
• Wolframite: Also heavy but typically forms bladed crystals

Distinguishing features include:

• Very high density
• Adamantine luster
• Brown to black coloration

Identification Notes for Collectors

• Look for heavy, brown to black crystals or botryoidal masses
• Check for adamantine luster and high density
• Note association with granite-related minerals like quartz and tourmaline
• Observe lack of strong cleavage
• Use density and hardness tests to distinguish from similar minerals

Cassiterite is the primary ore of tin, recognized for its high density, bright luster, and occurrence in hydrothermal and placer deposits, making it both economically important and desirable for collectors.